根据日期计算出周一和周日

stackoverflow上的回答

http://stackoverflow.com/questions/2821035/c-sharp-get-start-date-and-last-date-based-on-current-date

DateTime baseDate = DateTime.Today;

var today = baseDate;
var yesterday = baseDate.AddDays(-1);
var thisWeekStart = baseDate.AddDays(-(int)baseDate.DayOfWeek);
var thisWeekEnd = thisWeekStart.AddDays(7).AddSeconds(-1);
var lastWeekStart = thisWeekStart.AddDays(-7);
var lastWeekEnd = thisWeekStart.AddSeconds(-1);
var thisMonthStart = baseDate.AddDays(1 - baseDate.Day);
var thisMonthEnd = thisMonthStart.AddMonths(1).AddSeconds(-1);
var lastMonthStart = thisMonthStart.AddMonths(-1);
var lastMonthEnd = thisMonthStart.AddSeconds(-1);

DevExpress的横轴，根据时间，自适应

https://documentation.devexpress.com/#WindowsForms/CustomDocument6464

周一为每周的第一天，进行时间的推断

private static DayOfWeek firstDayOfWeek = DayOfWeek.Monday;

        public static DayOfWeek FirstDayOfWeek
        {
            get { return firstDayOfWeek;}
            set { firstDayOfWeek = value; }
        }

 public static DateTime GetWeekStart(this DateTime time)
        {
            DayOfWeek lastDayOfWeek;
            if (firstDayOfWeek == DayOfWeek.Sunday)
            {
                lastDayOfWeek = DayOfWeek.Saturday;
            }
            else
            {
                lastDayOfWeek = firstDayOfWeek - 1;
            }

            DateTime weekStart;
            if (time.DayOfWeek == lastDayOfWeek)
            {
                //用默认的第一天减去最后一天,相差6天
                weekStart = time.AddDays(DayOfWeek.Sunday - DayOfWeek.Saturday);
            }
            else
            {
                weekStart = time.AddDays(firstDayOfWeek - time.DayOfWeek);
            }
            return weekStart;
        }